1. Field of the Invention
The present invention relates to a thermal printer in which a thermal head provided with heating elements is electrically driven to generate heat from said heating elements, thereby printing a desired pattern on a heat-sensitive recording sheet or transferring heat-fusible ink of an ink ribbon in a desired pattern onto a recording sheet by means of said heat.
2. Description of the Prior Art
For use in the thermal printers there have conventionally been known a few types of thermal heads, such as a line printing head having a horizontal array of heating elements of a number equal to the number of dots over the entire printing width, or a serial printing head having a vertical array or a matrix of heating elements and moved in the direction of printing width.
FIG. 1 shows an example of such thermal head having a vertical array of seven heating elements prepared by thin film technology.
As shown in FIG. 1, a ceramic substrate 101 is provided thereon with a heat insulating layer 102 composed for example of a glass material, on which are formed resistors 103, for example of TaN. Except for heating element areas 103a, said resistors 103 are covered with conductor layers 104 composed for example of aluminum or gold and constituting lead wires thereby exhibiting a high resistance only in said heating element areas 103a. Thus a current supply for example between a terminal 105-1 and a common terminal C generates heat only in the uppermost heating element 103-1. A radiating plate 106 is provided for preventing heat accumulation in the ceramic substrate.
In the thermal printer, the thermal head as shown in FIG. 1 is pressed against a heat-sensitive recording sheet or against a recording sheet across a thermal transfer ink ribbon, and the desired heating elements of the thermal head are energized to generate heat when said thermal head reaches a desired position by the displacement of said head or said recording sheet, thereby generating color in said heat-sensitive recording sheet or transferring the heat-fusible ink of said ink ribbon in a desired form onto the recording sheet.
As will be apparent from the above-described structure, the thermal head has a certain heat capacity. Since the printing is achieved by heat generation from the heating elements, it is difficult to maintain the heating elements of the thermal head at a constant temperature from the start to the end of printing operation. Moreover, as the printing operation progresses the thermal head accumulates heat, thus causing higher printing density and resulting in uneven printing.
Also in the case of printing a line for example, the same heating elements are continuously energized over a long period and can become abnormally heated, so that the service life of the thermal head becomes shortened because of the fatigue caused by high temperature.